The present invention relates to a longitudinal plunging unit for a driveshaft assembly.
The invention relates to a longitudinal plunging unit for a shaft assembly used for torque transmitting purposes. Such plunging units include a profiled sleeve with circumferentially distributed, longitudinally extending first ball grooves, a profiled journal with circumferentially distributed, longitudinally extending second ball grooves, balls which are arranged in groups of balls in pairs of first and second ball grooves, and a ball cage which is positioned between the profiled sleeve and the profiled journal and which axially fixes the balls in their positions relative to one another.
From DE 199 52 245 A1, there is known a longitudinal plunging unit or telescopic shaft of the foregoing type wherein hardness distortions are avoided in that the number of the first ball grooves corresponds to a multiple of the number of the second ball grooves and wherein the excess part of the first ball grooves remains free of balls.
EP 0 189 011 A proposes a further longitudinal plunging unit of the foregoing type for torque transmitting purposes. Freedom from play in the direction of rotation is achieved in that at least one ball or roller comprises an excess dimension relative to the grooves and includes a resilient material that is fitted under pretension.
When, under torque, the profiled sleeve is displaced relative to the profiled journal, the balls roll in the inner and outer ball grooves in a way which is substantially free of sliding friction. Thus, the balls and the ball cage with all its balls, cover half the relative displacement path between the two elements, and permit the change in length of the longitudinal plunging unit in a low-friction way. At the ends of the displacement path, axial stops for the ball cage or for the balls are provided in the profiled sleeve or on the profiled journal. Under normal operating conditions, the ball cage should, if possible, not reach the axial stops, but, in accordance with the specified design, it should be freely displaceable, thus avoiding any sliding friction of the balls in the grooves.
Assemblies of this type are used in torque transmitting shafts, such as driveshafts. They are used for the purpose of compensating for tolerances of the distance between the connecting parts during assembly and/or for compensating for changes in the distance between the connecting parts, such as joints. These assemblies require low-friction plunging conditions during operation. The basic design of such longitudinal plunging units is known. During operation, they are subject to a problem in that, during the specified transmission of torque, there necessarily occurs torsion at the inter-engaging portions of the profiled sleeve and of the profiled journal in the region of the ball grooves. As a result, within the groups of balls, the balls of the units positioned in the longitudinal direction at the ends are subjected to the highest loads due to the greatest differences in torsion occurring in those regions. In consequence, there can occur damage such as pittings. This damage occurs at the balls at the ends first, and is the starting point of the failure of the entire unit.
It is an object of the present invention to provide a longitudinal plunging unit of the foregoing type which has improved running behavior and, thus, a longer service life.
According to a first solution, in the region of the ball grooves, the profiled sleeve and the profiled journal comprise the same torsional stiffness, such that during the introduction of torque, they are subject to the same angles of torsion. In particular, in the region of the ball grooves, the profiled sleeve and the profiled journal comprise the same modulus of transverse elasticity (modulus of rigidity) of the material. They can be made of identical materials, while at the same time they can have the same polar inertia moment. This solution constitutes a first measure of reducing, under torque conditions, the torsional differences (angular errors) which, if viewed from the center, increase in the longitudinal direction between the torsion of the profiled sleeve and thus of the outer ball grooves, and the torsion of the profiled journal and thus of the inner ball grooves, in order to ensure that the loads to which the balls are subjected under torque are as uniform as possible.
A second solution is provided wherein, within the groups of balls, the size of the balls varies in the longitudinal direction. The balls positioned centrally in the longitudinal direction have the greatest diameter and the balls positioned at the ends have the smallest diameter. In particular, the size of the balls decreases linearly from ball to ball from the center outwardly. However, it is also contemplated to provide a sub-group of central balls which are identical in size and which are adjoined on both sides by smaller balls. The size of the balls is preferably such that in the torque-free condition of the longitudinal plunging unit, only the central balls are radially pretensioned between the profiled sleeve and the profiled journal. With this solution, it is assumed that, when under the influence of a torsional load, the profiled sleeve and the profiled journal are subjected to increasing torsion starting from their respective free ends. In such a case, the introduction of torque via the balls has not yet much of an effect to the respective connected ends, so that the torsional differences (angular errors) between the two parts are greatest at the respective connected ends which introduce the torque. To ensure that the balls are subjected to a uniform load, in accordance with the invention, the ball size is reduced from the center to the respective ends. Best uniform contact, i.e. uniform ball forces at all balls, should be provided at the torque occurring most frequently in the load spectrum.
The solutions detailed herein are based on theoretical equations which act as models for tortional behavior of the plunging unit. To that extent, it is particularly advantageous to combine the two solutions.
Two preferred embodiments of the invention are illustrated in the drawings and will be described below. Other advantages and features of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.